The last several years has seen an increasing amount of emphasis on designing engines having improved fuel economy and efficiency, reduced emissions, a greater service life, and an increased power output per cylinder. The trend has resulted in increasingly more severe mechanical and thermal requirements on the piston member. The crown region of a piston member is heated by the burning fuel and air mixture. The piston assembly including the piston rings must make effective contact with the cylinder bore to prevent the egress of hot combustion gases and to control lubricating oil under all operating conditions. The temperature and combustion pressures on the piston member particularly must remain within prescribed material, structural and thermal limits or early failure will result.
The cooled composite piston assembly disclosed in U.S. Pat. No. 4,581,983 issued to H. Moebus on Apr. 15, 1986 is illustrative of one configuration that can withstand such increased power output levels. However, the upper and lower parts thereof are joined together by welding, and this is a costly process that is preferably to be avoided.
A more desirable type of piston assembly is disclosed in U.S. Pat. No. 4,056,044 issued to Kenneth R. Kamman on Nov. 1, 1977. The Kamman patent, which is assigned to the Assignee of the present invention, teaches the use of an articulated piston assembly having an upper piston member and a lower skirt which are individually pivotally connected to a common wrist pin. Oil directed to a trough in the skirt is advantageously splashed in a turbulent "cocktail shaker" action against a recess in the underside of the crown surface adjacent the ring grooves for cooling the interior of the piston. Subsequent extensive testing thereof with cast elements has indicated that the practical level of knowledge on casting procedures is insufficient to resist combustion pressures above about 13,790 kPa (2,000 psi). Specifically, an excessive number of the upper cast steel piston members had so much porosity that premature failure resulted. On the other hand, a few cast steel piston members were manufactured with relatively low levels of porosity so that they survived a relatively rigorous testing program. While extensive studies were conducted to minimize porosity levels in the cast members, the levels remain too high. One way to check for porosity is to fully x-ray piston, which not only is unacceptable from a cost stand point but also does not guarantee that the piston is totally free of porosity.
In addition to porosity considerations, it should be appreciated that the structural shape and strength of each element of an articulated piston assembly is in a continual stage of being modified to better resist higher compressive loads and thermally induced forces. For example, Society of Automotive Engineers, Inc. Paper No. 770031 authored by M. D. Roehrle, entitled "Pistons for High Output Diesel Engines", and presented circa Feb. 28, 1977, is indicative of the great number of laboratory tests conducted throughout the world on the individual elements. That paper also discusses a number of considerations to minimize cracking problems in light alloy or aluminum piston members resulting primarily from thermal constraints.
U.S. Pat. No. 4,662,047 issued to Rutger Berchem on May 5, 1987 discloses a one-piece piston produced by die pressing of a previously forged blank to bend an annular cylindrical collar thereon. A forged piston can offer the capability of resisting high combustion chamber pressures and temperatures; however, the forging of parts with relatively thin wall sections having extremely close dimensional tolerances and the forming of narrow and deep cavities having precise relative locations is very difficult, if not impossible. Therefore it is frequently the manufacturing tolerances that limit or prevent the forging of the thin wall sections and narrow deep cavities that are so desperately required for better heat dissipation. Complex shapes and varying wall thicknesses can also result in uneven heat distribution and differential thermal distortion of the piston, so another objective is to simplify the construction as much as possible including maximizing the symmetry thereof about the central axis.
Also, another problem to consider is that the relatively rough surface finish produced by the forging process can produce stress risers, and this is especially critical in the high load areas of the piston member such as in the thin wall sections and cavities. Oftentimes these crack propagation areas are undetectable with disastrous results.
Thus, what is needed is a high output engine piston assembly having a piston member therefor which is capable of continuous and efficient operation at combustion chamber pressures above about 13,790 kPa (2,000 psi), and preferably in the region of about 15,170 kPa (2,200 psi). Furthermore, the piston member should preferably be forged from an alloy steel material having a configuration substantially devoid of complex shapes to allow the forging thereof. Moreover, the region of the upper portion of the piston member and specifically the cooling recess region should preferably have relatively thin, substantially constant wall thicknesses for substantially even heat distribution and for maximum cooling of the surfaces. Also, the surfaces of the cooling recess should be machined surfaces of revolution for precise dimension control between adjacent surfaces and especially between the cooling channel and the ring grooves. The piston member should preferably include symmetrical surfaces of revolution about the central axis with the surfaces being free of imperfections that could cause the propagation of cracks and so that differential thermal distortion can be avoided.
The present invention is directed to overcoming one or more of the problems as set forth above.